This invention relates generally to headphones and more particularly to a headphone for reproduction of sound with a frequency response which is analogous to that provided by loudspeakers, and still more particularly to a headphone with a sound pressure vs. frequency response having two resonant peaks to a listener's eardrum.
The following is a discussion of the significance of the sound pressure vs. frequency characteristic of a headphone to such realism as occurs in a loudspeaker system.
FIG. 1 depicts a diagram of a loudspeaker sound pressure measuring system using a dummy head 1. The dummy head 1, which duplicates a human head in dimensions, is positioned at an angle of B 30 degrees to a loudspeaker 3 at a distance of 2 meters therefrom. Although the speaker itself has an essentially flat frequency response and a linear phase response, the response at the dummy head 1 exhibits two resonant peaks in the frequency response curve as shown in FIG. 2. Our investigation has revealed that the first peak is caused by resonances in the ear canal, while the second peak is due to diffractions from the head and external ear.
FIG. 3 depicts a diagram illustrating a dummy head 1 equipped with a headphone 4. If the sound pressure vs. frequency characteristic as shown iN FIG. 2 is reproduced at the diaphragm of the microphone 2, such a headphone imparts the sensation of realism to a listener as if he is hearing sound from the loudspeaker.
FIG. 4A depicts a vertical cross sectional view of a conventional headphone, and FIG. 4B depicts an enlarged sectional view of a portion of the headset of FIG. 4A. A diaphragm 9, mounted within a housing 5, defines a rear cavity 12 with the rear wall of the housing 5. The housing 5 is formed with an aperture 13 in which a mesh 13' is fitted. The diaphragm 9 is supported by suitable spacers 8 and carries a printed voice coil (no numeral) made of, for example, aluminum. An earpad 7 is secured to one side of front panel 6 with an opening (no numeral). The front panel 6 is in turn fixedly attached at its other side to the housing. A pair of yokes 11 are retained within the housing 5 in a manner to hold the spacers 8 therebetween. Each of the yokes 11 carries a plurality of plate-like magnets 10 thereon. The arrangement of the magnets 10 is such that the polarities of the facing magnets are identical with each other.
With this arrangement, when a current is supplied to the voice coil, fluxes of the magnets 10 cause the diaphragm 9 to vibrate in accordance with Fleming's left hand law. However, in sound pressure vs. frequency response of the conventional headphone of FIG. 4A of the conventional headphone measured with the system of FIG. 3, only one resonant peak occurs as indicated by broken line curve (a) of FIG. 7. This means that the sensation of realism identical to that of the loudspeaker system is not achieved with this prior headphone.